The present invention relates to the field of calibration. More specifically, the invention relates to calibrating analog-to-digital converters.
Conventional analog-to-digital (A/D) converters accumulate errors during conversion of analog signals into digital format. For example, the precision of the components in A/D converters varies, which leads to errors in the conversion process. These errors are typically corrected by calibration. A disadvantage of conventional calibration is the requirement for control logics dedicated to the calibration process, which increases the cost and complexity of conventional A/D converters.
Conventional calibration processes may also utilize a separate calibration standard, such as an externally applied calibration voltage, in order to calibrate an A/D converter. However, calibration results in some converters, such as multislope converters, based on externally applied calibration voltages, do not reflect the errors that actually occur in an A/D converter during the conversion process. Also, in conventional multistage converters, a separate reference is required for each stage of the converter, which leads to further inaccuracies.
Therefore, a need exists for a calibration method for A/D converters that accurately reflects errors that occur during conversion of analog signals into digital format. A need also exists for a calibration method of reduced cost and complexity.
The present invention satisfies the above needs and achieves other advantages not present in conventional devices.
According to an aspect of the invention, an A/D converter is operable in either an analog-to-digital conversion mode, or in a calibration mode. In the conversion mode, the A/D converter performs conversion processes during which analog input signals are converted into digital output signals. In the calibration mode, the A/D converter performs a calibration process. During the calibration process, the A/D converter calculates a calibration factor. The calibration factor is used in conversion processes to correct for errors in the conversion processes.
The calibration process includes the steps of performing a plurality of first calibration cycles, and then performing a plurality of second calibration cycles. The plurality of first calibration cycles include the steps of applying a secondary discharge current to an integrator for a first calibration time, and applying a primary discharge current to the integrator for a first discharge time. A first discharge value is determined from the first calibration cycles. The plurality of second calibration cycles also use the primary discharge current and the secondary discharge current, in order to determine a second discharge value. A calibration factor is calculated from the first and second discharge values.
According to the above aspect, the first and second discharge values are a function of the ratio of the primary discharge current to the secondary discharge current. Therefore, an external reference is not required. In addition, it is not necessary to calculate the actual values of the primary and secondary discharge currents, or of the corresponding reference voltages used to generate the primary and secondary discharge currents, because only the ratio between the values is required to calculate the calibration ratio.
As a further advantage, the reference voltages may be the same references used during conversion operations of the A/D converter. Therefore, the calibration result accurately reflects the errors arising during conversion operations of the A/D converter.
Other aspects and advantages of the invention will be discussed with reference to the figures and to the detailed description of the preferred embodiments.